title: Conformational changes of the S2YZ* intermediate of the S2 to S3 transition in photosystem II.
abstract: The paper extends earlier studies on the S(2)Y(Z)* intermediate that is trapped by illumination in the temperature range 77 K to 190 K of untreated samples poised in the S(2)...Q(A) state. X-band EPR experiments on untreated and glycerol (50% v/v) treated samples at 10 K indicate that the intermediate consists of two components. A wide one with a splitting of ca 170 G, and a narrow one characterized by a splitting of ca 120 G (untreated), or 124 G (glycerol-treated samples). Lower temperatures of illumination in the above temperature range favor the wide component, which at 10 K decays faster than the narrow one. Re-illumination at 10 K after decay of the signal trapped at 77-190 K induces only the narrow component. Rapid scan experiments in the temperature range 77-190 K reveal high resolution spectra of the isolated tyz Z* radical and no evidence of alternative radicals. The two split signals are accordingly assigned to different conformations of the S(2)Y(Z)* intermediate A point-dipole simulation of the spectra yields "effective distances" between the spin densities of Y(Z)* and the Mn(4)Ca center of 5.7 Å for the wide and 6.4 Å for the narrow component. The results are discussed on the basis of a molecular model assuming two sequential proton transfers during oxidation of tyr Z. The wide component is assigned to a transient S(2)Y(Z)* conformation, that forms during the primary proton transfer.
authors.affiliation: Institute of Materials Science, NCSR Demokritos, Athens 15310, Greece.
authors.forename: Maria
authors.initials: M
authors.lastname: Chrysina
authors.forename: Georgia
authors.initials: G
authors.lastname: Zahariou
authors.forename: Yiannis
authors.initials: Y
authors.lastname: Sanakis
authors.forename: Nikolaos
authors.initials: N
authors.lastname: Ioannidis
authors.forename: Vasili
authors.initials: V
authors.lastname: Petrouleas
mesh_terms: D020134:Catalytic Domain; D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals / Q000737:chemistry; D007700:Kinetics; D045332:Photosystem II Protein Complex / Q000737:chemistry* / Q000378:metabolism; D013696:Temperature; D014443:Tyrosine / Q000737:chemistry
chemical_list: D005609:Free Radicals; D045332:Photosystem II Protein Complex; D014443:Tyrosine